چکیده انگلیسی

Purpose
Previous studies have produced conflicting results with regard to the association between birth weight and developmental stuttering. This study sought to determine whether birth weight was associated with childhood and/or adolescent stuttering in three British birth cohort samples.
Methods
Logistic regression analyses were carried out on data from the Millenium Cohort Study (MCS), British Cohort Study (BCS70) and National Child Development Study (NCDS), whose initial cohorts comprised over 56,000 individuals. The outcome variables were parent-reported stuttering in childhood or in adolescence; the predictors, based on prior research, were birth weight, sex, multiple birth status, vocabulary score and mother's level of education. Birth weight was analysed both as a categorical variable (low birth weight, <2500 g; normal range; high birth weight, ≥4000 g) and as a continuous variable. Separate analyses were carried out to determine the impact of birth weight and the other predictors on stuttering during childhood (age 3, 5 and 7 and MCS, BCS70 and NCDS, respectively) or at age 16, when developmental stuttering is likely to be persistent.
Results
None of the multivariate analyses revealed an association between birth weight and parent-reported stuttering. Sex was a significant predictor of stuttering in all the analyses, with males 1.6–3.6 times more likely than females to stutter.
Conclusion
Our results suggest that birth weight is not a clinically useful predictor of childhood or persistent stuttering.

مقدمه انگلیسی

Many studies have attempted to identify risk factors associated with stuttering; see, for example, Andrews and Harris (1964), Ardila, Bateman, and Niño (1994), Berry, 1938a and Berry, 1938b, Cook, Howell, and Donlan, (2013), Craig, Hancock, Tran, Craig, and Peters (2002), Howell (2013), Howell and Davis (2011), Johnson (1955), Månsson (2000), Reilly et al. (2009) and Yairi and Ambrose (2013). Packman (2012) has proposed a model of developmental stuttering whose central hypothesis is that the fundamental cause of the disorder is a neural deficit. This hypothesis is consistent with the findings of many recent studies, which have reported structural and functional abnormalities in the brains of people who stutter (Chang et al., 2008, Chang et al., 2011, Cykowski et al., 2010 and Watkins et al., 2008). One risk factor for abnormal neural development is birth weight (Walhovd, Fjell, & Brown, 2012). Low birth weight is well documented as a major determinant of mortality, morbidity and disability in infancy and childhood and also has a long-term impact on health outcomes in adult life (World Health Organization, 1992). High birth weight can be associated with complications during childbirth (Zhang et al., 2008). Either of these variations might have an impact on neurological development that could lead to stuttering.
Two recent studies have reported conflicting results regarding the association between birth weight and developmental stuttering. Reilly et al. (2009) used a birth cohort sample of 1612 children to identify the predictors of therapist-diagnosed stuttering in one hundred and fifty eight 3-year-olds. As well as birth weight, they examined the influence of child's temperament, language development, maternal mental health, maternal education, sex, premature birth status, birth order, twinning, socio-economic status, and family history of stuttering. They found that cohort members (CMs) who stuttered at age 3 were significantly more likely than other CMs to be male, be a twin, have a high vocabulary score at 2 years of age, and be the child of a highly-educated mother. Birth weight was not a significant predictor of stuttering in their study.
By contrast, Boulet, Schieve, and Boyle (2011) did find an association between birth weight and stuttering. They examined the relationship between birth weight and several developmental disorders, including stuttering, in a US parent survey that studied 87,578 children of ages 3–17 years; approximately 25% were 3–5 years old. The parent-reported data included the child's birth weight and whether the child had stuttered during the previous year. In their analyses of all of the developmental disorders, the authors adjusted for the same factors: age, sex, race, household income, maternal education and year of survey. Using birth weights of 3500–3999 g as the reference category, they found that birth weights up to 2999 g were between 1.3 and 3.0 times more likely to be associated with stuttering.
One difference between the two studies that might explain this discrepancy relates to the different age ranges of the participants in the two studies and potential differences between children who recover from stuttering and those whose stuttering is persistent. Perhaps birth weight is only a predictor of persistent developmental stuttering. Since there is a high rate of spontaneous recovery among pre-schoolers who stutter (Ambrose et al., 1997 and Yairi et al., 1996), the majority of the children in Reilly et al.’s (2009) sample would be likely to recover. Boulet et al.’s (2011) much larger sample included participants in their teens; since most, if not all, of these older participants who stuttered would belong in the persistent category, Boulet et al's sample could contain a higher proportion of participants with persistent stuttering than Reilly et al's. Unfortunately, the way in which Boulet et al. report the data does not allow this possibility to be examined.
The aim of the present study was to determine the relationship between birth weight and developmental stuttering in three British birth cohort data sets, which contain data from over 56,000 participants in total. Using binary logistic regression analyses, we attempted to control for the factors that Reilly et al. (2009) found to be significant predictors of stuttering. We first considered the relationship between birth weight plus these other factors and parent-reported stuttering during childhood. We then considered these factors in CMs at age 16, comparing those who were still reported to stutter at this age with those who had never been reported to stutter. Based on the reasoning in the previous paragraph, we hypothesised that birth weight would be significantly associated with developmental stuttering at age 16 but not during childhood.